A steering system for steering a vehicle includes a pinion gear on the side of a steering shaft and a rack bar on which rack teeth are formed on the side of tie-rods which connect left and right, front road wheels. Then, rotational steering force transmitted from a steering wheel is converted to a horizontal lateral force by a steering gear box, and the horizontal lateral force is then transmitted to the mad wheels by way of the pinion gear and the rack teeth, whereby a rotational force around king pins is applied to the road wheels (see, e.g., JP2007-253190A).
On the rack bar, the steering force transmitted from the steering wheel is transmitted from contact faces of the pinion gear and the rack teeth. Consequently, the larger the contact faces, are the more efficiently the steering force can be transmitted.
In addition, as a method for producing a hollow rack bar, a method is known for rolling and forging as cylindrical metal member from an inside thereof. In this method, a mandrel is press fitted in an interior of the cylindrical metal member so as to a metal material to flow into recess portions of a die to thereby form rack teeth (see, e.g., JP2008-264874A).
The rack bar described above has the following problem. Namely, as a method for forming a rack bar, there is known a method for press forging a pipe member 410 as shown in FIG. 8. The pipe member 410 is fixed in place by molds K1 and K2. A central portion of the pipe member 410 is collapsed flat into a central recess portion in advance by using a separate die, and a teeth forming die 430 is brought into abutment with this central recess portion 405. In this method, a mandrel T having a tapered projection T1 on an upper surface side thereof is press fitted into the pipe member 410. A material of the central recess portion 405 is caused to project into recess portions of the teeth forming die 430 by the projection T1 on the mandrel T. Thereafter, the mandrel T is replaced with a mandrel having a slightly larger projection T1 and the replacement mandrel is then press fitted into the pipe member 410. By repeating this process, the material of the central recess portion 405 projects progressively, whereby rack teeth are formed.
As shown in FIGS. 9 and 10, when a rack bar 411 is formed which has rack teeth which are formed over the whole area of the central recess portion 405 of the pipe member 410 in an axial direction thereof, rack teeth which are smaller than those at a central portion are formed at portions lying near axial end portions of a rack toothed portion LB1. For example, as to rack teeth H1, H2, materials in ranges L1, L2 project into the recess portions of the teeth forming die 430 when the pipe member 410 is rolled and forged, whereby the rack teeth H1, H2 are formed. The materials supplied from the ranges L1, L2 are not sufficient to fill the recess portions of the teeth forming die 430, and therefore, the rack teeth H1, H2 are formed smaller than other rack teeth. The rack teeth H1, H2 which are formed in this way do not have required strength and durability and steering force transmission efficiency with the pinion gear. From the reason described above, the rack teeth lying near the axial end portions are formed smaller than the other rack teeth. Because of this, the strength and durability and steering force transmission efficiency with the pinion gear of the rack teeth are not sufficient at the portions lying near the axial end portions of the rack toothed portion.
In contrast with this, as shown in FIGS. 11 and 12, when a rack bar 412 is formed which has flat portions F1 near end portions of a central recess portion 405 of a pipe member 410, even at end portions of a rack toothed portion LB, a material of which a rack tooth H3 is formed can be supplied sufficiently from the flat portion F1. Because of this, the rack tooth H3 having the sufficient strength and durability and steering, force transmission efficiency with the pinion gear can be formed over the Whole area of the rack toothed portion LB2. On the other hand, the thickness of the flat portion F1 is reduced because the material thereof is supplied to the rack toothed portion LB2. The strength and durability of the flat portion F1 whose thickness is so reduced are reduced, and therefore, the strength and durability of the rack bar itself become insufficient.